Passive two beam interferometry employs an N×N coupler to split and recombine the beams. The object of two beam interferometry is to measure the phase difference between the two interfering beams, which in turn becomes a measure for a quantity which is to be measured in various measurement fields including e.g. optical gyroscopes and current measuring devices. With N>2, it is possible to avoid the situation in which the output intensities are simultaneously stationary with respect to the phase difference, and it is thus unnecessary to use an active phase shifter, hence the name passive interferometry.
Passive interferometry may be preferable to active interferometry in many applications, for example if the interferometer head is at high voltage.
In interrogating the N×N coupler of a passive two beam interferometer, typically a pulsed light source is used for inserting optical signals into the N×N coupler. A suitable optical network splits each signal pulse from the pulse source into N pulses, one for each of the N input ports of the coupler.
Each one of the N input pulses gives rise to N output pulses, giving N2 output pulses in all, and the delays in the optical network are typically such that the N2 pulses arrive at different times at a detector, and can therefore be distinguished.
A light pulse from the source reaches each of the ports of the coupler with some loss of intensity, which in general will be in part due to variations in the source power relative to a nominal source power. Similarly, a light pulse leaving a port of the coupler reaches the detector with a further loss factor, which also will in general be in part due to variations in the detector sensitivity relative to a nominal value for this sensitivity. Furthermore, the interferometer head is characterised by values of intensity transfer factors from each input port to each output port.
In order to obtain useful measurements utilizing passive two beam interferometry, it has thus far been necessary to conduct a temperature dependent calibration process to determine the effect of loss factors and intensity transfer factors. This has the disadvantages of(a) additional i.e. calibration processing steps being required in utilizing passive interferometry in the various measurement fields, and (b) the necessity to repeat calibration periodically to account for changes in e.g. the environment in which the passive interferometer is used.
In at least preferred embodiments, the present invention seeks to provide an improved signal processing for passive interferometry which can avoid those disadvantages.